The theory is based around Ibn Khaldun’s concept of asabiyyah or “social cohesion”. The ability to cooperate effectively is fundamental to imperial nations. Turchin argues that this arises on metaethnic boundaries (faultlines between groupings of ethnicities) in situations of intense competition. Once high levels of asabiyyah have been achieved, they can persist for centuries. But imperial success then sets in pace processes by which asabiyyah declines. Once it has collapsed, the lack can persist for centuries.

That empires are founded by militarily effective folk and militarily effective folk arise in areas of lots of prolonged military activity is hardly a startling claim. Nor is that cooperation is necessary for military effectiveness. Still less that more ferocious selection mechanisms can lead to creation of effective predators. What makes Turchin’s analysis striking is his analysis of the demographic dynamics of disintegration and reintegration in agrarian societies.

In agrarian societies, imperial success means rising population. As population rises, inequality increases (since, once the advantages of scale and scope for a given level of technology have been captured, land is a basic constraint). The social middle collapses, as land holdings are divided and fall below the level able to sustain those living off it. Land is sold to pay for food. It is sold to the wealthy who accumulate increasing assets. The increasing inequality undermines asabiyyah. If not checked by an external threat forcing reordering of the social contract to face it, the process continues until asabiyyah collapses and the Imperial structure along with it.

Within the grand cycles of imperial rise and fall, there are smaller cycles of integration and disintegration driven by changes in the elite. Imperial success means the population of the elite rises faster than the means to sustain them in the style they aspire to. This leads to intense competition within the elite up to the point of civil war: if that conflict “thins out” the elite enough (without destroying the imperial structure itself), the disintegrative phase may be followed by an integrative one. Even within the disintegrative phases, there are “father-and-son” cycles, where the father’s generation fights bitterly, their sons’ seek to avoid the mistakes that lead to violence, but the underlying pressures still exist, so their grandsons repeat the pattern as memories of the causes and consequence of civil strife fade.

Peter Turchin backs up his theory with a series of case studies. His discussion of Roman and medieval French history (with asides to medieval English history) were very revealing and full of useful and striking information. (The sort of detail that social analysts love but historians often leave out.) Since I had already in part analysed “political correctness” as a manifestation of elite competition, it was a bit startling to read about a much wider analysis of the patterns of such things.

In order to make his theory work, Turchin has to rehabilitate a scientific basis for human cooperation. As he points out, the overwhelming trend of social and biological analysis has been towards individual action. But, he argues, rational choice theory cannot account for the levels of cooperation we clearly observe. At first I thought we were going to get another tiresome straw-person dismissal of economics. Far from it: he concedes the power of rational choice theory—indeed, uses it himself. (His secular cycles are all about changing behaviour from changing incentives.) Turchin just believes it needs to be supplemented by the growing empirical data on human cooperative behaviours. Cross-cultural experimental data suggest that humans divide into “knaves” (always self-interested), “saints” (always moralistic) and “moralists” (moral with punishment). Turchin argues that asabiyyah rises in a situation where moralists can be effective and collapses when they aren’t.

One of the points in Turchin’s favour is that things that struck me as dubious or unexplained when I first read them get dealt with later in the book. For example, he argues that the core regions of the Roman Empire became, and remained asabiyyah “black holes”, particularly Southern Italy. My immediate reaction was: Why not Sicily? Plenty of conflict and frontier activity. But his argument is more sophisticated—that any revitalisation was overwhelmingly dealing with small elites (such as the Normans) who, once they get absorbed into the local population, replicate its (notoriously) uncooperative social patterns. He also argues that the socially disintegrative effects of mass slavery can persist long after the institution has been abolished.

He is very much of the view that the Byzantine Empire is such a different beast from the preceding Roman Empire: that it should be understood as, effectively, a new Empire. Indeed, he regards the Empire as not being Roman in any useful sense after the crisis of the third century. Such relabelling to fit theory always makes me nervous. On the other hand, the Dominate of the later Empire was a rather different beast than the Principate early Empire.

But his use of historical data rang true to me, and I certainly didn't catch any howlers (apart from one passage that used Dominate and Principate the wrong way round). Indeed, his suggestion that persistent Chinese imperial unity (which, as he points out, is very unusual among human societies) was a result of nomad pressure is certainly a striking thought.

One of the welcome aspects of his approach is that he is very cautious in applying it forward. As he points out, modernity is very different from agrarian societies. He notes some continuing patterns (such as oscillating cycles of equality and inequality, and rising and falling crime rates) but is not willing to go beyond that. He does argue for the power of underlying social patterns, which is reasonable.

The book is certainly engagingly written and full of useful information. Applying broad demographic patterns intelligently to history is a worthy exercise. No doubt, critics will start to pick away at his thesis. (Or otherwise consider it, as in this thoughtful review from someone with a genetics and population dynamics background.) I am neither entirely convinced nor dismissive: a very thought-provoking book.